As far as I know, most older saints were granted fantasy coats of arms in the
Middle Ages (even God and the Devil got them!), so at least banners of arms
could have been created from them. I'm not learned on the subject but I know at
least that main personas of the Christian "pantheon" have regularly used
colors; blue and silver for the Virgin Mary (or is it gold?) and red and
green for Saint John -- these last ones being the remote cause for the current
Portuguese flag colors.

These medieval coats of arms also influenced at
least some later heraldic devices of Military Orders in saint's names. Today I'm
sending putative banners of arms of some of them. António
Martins, 19 February 1998

In the early, less regulated days of heraldry, the designers rather took the
bit between their teeth and galloped off in all directions at once. Important
people in their own time all had coats of arms, therefore it was only common
sense that the even more important people in the past also had them. St
Wilfred's arms are seen in York Minster and St Wilfred's Church, three gold suns
on blue. St Wilfred lived five hundred years before heraldry was invented! The
early heralds even assigned arms to Christ and to Adam, although I doubt if even
they went as far as to assign them to God and Satan. Michael
Faul, 5 October 2001

In Christian symbolism several saints are associated with specific crosses,
either of a specific colour and/or of a specific shape.

The use of specific colours for specific saints originated from the British
Isles to mirror the case of Saint George (as used by England). As a result, it's
only in traditions originating there that "A Cross of Saint Andrew" is enough to
indicate both shape and colour (and colour of the background). Elsewhere this
would have to be "A white Cross of Saint Andrew (on a blue field)". Though
the Cross of Saint George has always been a Red Cross throughout on White, in
the British Isles, under the influence of the Union Jack, it has become limited
to symmetrical crosses, and this same limit applies for the other similarly
shaped crosses. The result of this development is that any symmetrical cross is
now sometimes defined as "A Saint George cross of such on such colours". All
those coloured crosses of the British Isles are crosses of Martyrs, with one
exception. This is caused by the fact that the original Cross of Saint Patrick
was a cross with a specific shape instead. The modern Cross of Saint Patrick was
created to fit in with the Cross of Saint George and the Cross of Saint Andrew
to allow adding it to the Union Jack. Patrick was not a Martyr. Peter
Hans van den Muijzenberg, 17 June 2001

Is this flag truly fixed? Someone on the Francovex list recently talked about
the French-used white cross on blue as being the St-Michel cross so I'm
wondering if either one is mistaken or the both combinations can be accepted
(such as the Scottish and Russian saltire both being referred to as
St-Andrew).Marc Pasquin, 11 July 2004

On the
Renouveau français website a flag with a white cross on blue is seen and
identified as being derived from the banner of St Michael.Tomislav
Todorovic, 18 May 2016

The above source was from a Masonic site, which suggests a limited use of
this design for the banner, i.e. within the Freemasonry. It shall be noted that
many of higher Masonic degrees are modelled after the chivalric orders and other
armigerous bodies, which extends to the symbols used, but those are always
modified by the Freemasons - use of unchanged symbols would certainly be
illegal. Consequently, it is quite possible that the banner of St Michael with
blue cross on white field is one of such modifications, created for the use by
Freemasons only.Tomislav Todorovic, 7 March 2017

Evidence that a white cross on blue field was indeed used as the symbol of St
Michael is the Tapestry of the Winged Harts (French: Tapisserie des Cerfs
ailés). This tapestry, which is nowadays kept at the Departmental Museum of
Antiquities, Rouen, France, was created in the 15th century, to commemorate
either the eventual French victory in the Hundred Years' War [1] or the founding
of the Order of St Michael [2]. The first of these events would put the creation
date [1] between the Battle of Castillon (1453) and death of Charles VII (1461),
while the latter [2, 3] would put it between 1469 (the Order's founding year)
and death of Louis XI (1483). The tapestry depicts three winged harts, the
animals which served as the badge of French kings [1]; two of them bear collars
in form of the crown with pendants in form of the royal shield of arms (azure
three fleurs-de-lys Or) and are entering an enclosure, where the third of them
is guarding a long swallow-tailed flag edged with gold fringes, which is
described as that of the Order of St Michael [2] and could be blazoned as: gules
semy of estoiles Or, over all St Michael charging at the dragon, both proper.
Depicted as a knight, St Michael is given a proper heraldic shield - azure a
cross argent. Two winged harts entering the enclosure are thought to symbolize
Normandy and Guyenne, brought back to French rule after the battles of Formigny
(1450) and Castillon (1453) [1], while the one which is already there would
symbolize France herself, St Michael being her patron; this veneration [1] owed
much to the fact that the Abbey of Mont Saint Michel had never been conquered by
the English during the Hundred Years' War, which is why it was eventually chosen
as the founding site and official seat of the Order of St Michael [1, 3]. To
complete the allegory, there are two lions outside the enclosure, obviously
unable to enter, symbolizing the defeated English [1].

This tapestry
reveals a blue cross on white field as the (banner of) arms of St Michael, but
also the origin of white cross as a much used symbol in the flags of Ancient
Regime.